Tags
biology, Bioprecipitation, Cloud, Ice, Ice crystals, Ice nucleus, Nucleation, Popular science, science, Water cycle
Strange as it may seem, water doesn’t actually freeze at zero degrees. In fact, even at temperatures as cold as -10°C, water still needs help turning into ice. Living creatures of all stripes have learned to take advantage of this curious fact in different ways, though none have done so with quite as much style as bacteria.
It seems self-evident that water would freeze at “freezing” temperatures, but this is only because it’s usually full of impurities. Ice, like any other crystal, forms more easily around a nucleus. A crystal is a bit like a molecular jigsaw puzzle — all of the pieces have to fit together just right. Just as puzzles are easier to continue than to start, crystals grow more easily from an initial core — the nucleus. This acts as a template, limiting the possible arrangements for the surrounding molecules. By doing so, it provides a pattern around which the crystal can coalesce. Pure water — really pure water — lacks anything around which the water molecules could organize, so it has to be cooled to an incredible -48°C before the molecules will slow down enough to form a crystal — that is, to freeze!
Water is normally full of nucleators, microscopic particles like dust and soot that can act as an ice nucleus. These aren’t all equally effective, though; just as some parts of a jigsaw make better starting points than others (perhaps because they’re more visually distinctive or detailed), some nucleators are better at arranging the water molecules into ice crystals. Though mineral and organic dusts are very common, they’re not very good at helping ice crystals form at temperatures warmer than around -15°C. In 1970, Russell Schnell was studying some very potent ice nucleators found in decaying plant matter and made the surprising discovery that they came from microbes. A few years later, Leroy Maki identified the bacteria Pseudomonas syringae as the source of these nucleators; at the same time, Deane Arny discovered that more frost formed on plants infected with P. syringae. The bacteria produce a special protein, InaZ, which can act as an ice nucleus at the relatively warm temperature of -2°C, probably because its repetitive shape is just right for coaxing water molecules into a crystalline arrangement. Researchers think that causing frost damage on plants may give the bacteria better access to nutrients. Ice-nucleating proteins have since been found in a wide range of organisms, from gallflies and gastropods to frogs. In some cases, this may be an adaptation to protect the animal by forming ice where it won’t cause harm; in others, the ice nucleation is probably just a side effect of the protein’s shape.
In 1982, David Sands suggested the intriguing possibility that bacteria might cause rain and snow. Air, including clouds, is usually full of micro-organisms like bacteria and fungi, some of which produce ice-nucleators. Ice crystals which form in clouds will grow until they are big enough to fall as either rain or snow depending on whether they melt on the way down. Bioprecipitation may sound like a far-fetched idea, but researchers have detected P. syringae in fresh rain, snow and ice from a wide range of locations including Louisiana, the French Alps and even Antarctica! Another team of researchers found that one-third of the ice crystals in clouds over Wyoming had formed around biological particles. Scientists have even been able to discover that the strains of P. syringae in rain falling over a soy bean field were different from those on the leaves, which means they probably came from somewhere else. These bacteria might be creating rain to help them travel long distances!
So does the fact that bacteria can cause rain have an impact on the global environment? Two different research groups added bacterial ice-nucleation to global climate models and found that it didn’t have a significant effect, though the researchers caution that they had to estimate a lot of parameters because of missing data. Regardless of their global impact, it seems clear that P. syringae has an effect on the local water cycle, which may even play a role in its life cycle. Whenever I’ve been asked for an example of other animals manipulating their environment the way we do, I’ve answered by pointing out the dams beavers build and the exquisite climate-control in ant nests. Next time someone asks, I’m going to tell them about these little critters, who control the weather to get around. I’m going to tell them how bacteria can make it rain.
Refs
Christner, B (2012) Cloudy with a chance of microbes Microbe
Constantinidou HA, Hirano SS, Baker LS, & Upper CD (1990). Atmospheric Dispersal of Ice Nucleation-Active Bacteria: The Role of Rain Phytopathology (80), 934-937 DOI: 10.1094/Phyto-80-934
Hoose C, Kristjánsson JE, & Burrows SM (2010). How important is biological ice nucleation in clouds on a global scale? Environmental Research Letters, 5 (2) DOI: 10.1088/1748-9326/5/2/024009
Morris CE, Sands DC, Vinatzer BA, Glaux C, Guilbaud C, Buffière A, Yan S, Dominguez H, & Thompson BM (2008). The life history of the plant pathogen Pseudomonas syringae is linked to the water cycle. The ISME journal, 2 (3), 321-34 PMID: 18185595
Lundheim R (2002). Physiological and ecological significance of biological ice nucleators. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 357 (1423), 937-43 PMID: 12171657
Pratt KA, DeMott PJ, French JR, Wang Z, Westphal DL, Heymsfield AJ, Twohy CH, Prenni AJ, & Prather KA (2009). In situ detection of biological particles in cloud ice-crystals Nature Geoscience, 2, 398-401 DOI: 10.1038/ngeo521
Sesartic A., Lohmann U., & Storelvmo T. (2011). Bacteria in the ECHAM5-HAM global climate model Atmos. Chem. Phys. Discuss., 11, 1457-1488 DOI: 10.5194/acpd-11-1457-2011
Jo Ann said:
How intriguing! I had no idea that water (pure water) wouldn’t freeze at temps we normally assume cause water to change to ice.
sedeer said:
I hadn’t quite grasped how cold water could get before researching this post. The world is full of amazing things! 🙂
Khalil A said:
Very interesting Sedeer. I’m wondering though, since water freezes thanks in large part to nucleators, then surely the amount of nucleators present is important as well. But 0C is regarded by the scientific community as freezing point of water so does this mean that 0C is different in different places because of different water contamination?
sedeer said:
Hi Khalil; thanks for the excellent question! I also remember learning that 0°C is defined as the freezing point of water, but apparently it’s actually defined as the “triple point” of water — that is, the temperature at which the solid, liquid and gas phases exist in equilibrium (at a particular (very low) pressure). In more general terms, the “freezing point” of a substance is usually determined by measuring the temperature at which the solid (crystal) form becomes liquid (ie, the melting point) since that process doesn’t rely on nucleation.
Interesting aside: it turns out there’s even a “standard” water used for the definition: “Vienna Standard Ocean Mean Water“. Who’da thunk it?
Thanks for stopping by!
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Achufusi Chinenye Emelda said:
how then do we know the particular rain this bacterium causes? is that the rain will have a particular smell or something, please let me know. and does it counteract what i was taught in geography about how rain is formed?
sedeer said:
Thanks for your questions. I don’t know of any reason why rain that was seeded by bacteria would seem different (in terms of smell or otherwise) from rain seeded by dust or soot.
It’s also not likely that this counteracts what you’ve been taught about how rain forms. Bacteria don’t fundamentally change the water cycle, but rather act as an extra element in it. They simply provide one more way for the water in the clouds to condense and form rain.
I hope that helps! If not, feel free to ask more questions!
Mark vann said:
If rain droplets and ice form around small particles, then can’t those particles be composed of anything, including atomic waste or biological contaminants? Acid rain may not be an accident, it might be ground up acidic chemical compounds, spread by aircraft. As a military weapon, this method might be invaluable. Electronic equipment could be infected with tiny microchips, which fall as rain, (or even better), as snow. Those microchips then begin to transmit messages through any electronic sensor sensitive enough to pick them up. The computers and microchips controlling those devices are then infected, and the new commands begin to take effect inside the mirochips, and the BIOS system.
sedeer said:
As far as I know, existing electronic equipment is far too big to serve as a nucleus for ice formation. Acid rain doesn’t form through nucleation; it forms when certain gases react with water to produce acid. And it certainly isn’t an accident, but it’s not the result of some conspiracy either — we’re just knowingly spewing large quantities of nitrogen and sulphur compounds into the air from our cars and our power plants. (To be clear, acid rain also results from natural processes that produce these gases, such as volcanoes and various biological processes.)
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Mahendra Pawar said:
What is the benefit of ice nucleation to the bacteria? Was this process evolutionary or present from the beginning?